The present application claims priority to United Kingdom patent application number 0110712.7, entitled xe2x80x9cSwitch Mechanismxe2x80x9d and filed on May 1, 2001.
The present invention relates to a switch mechanism and more particularly, but not exclusively, to a switch mechanism which can be operated by a rope to control the supply of power to for example kinetic machinery.
Known rope operated switch mechanisms comprise a safety switch adapted to be fitted in proximity to a machine and an actuator connected to the switch an operable by a rope to turn off the electrical power supply when the rope is pulled or slackened. Safety switches of this type have a housing in which a switch is mounted, the switch being actuable to switch between first and second conditions, the first condition corresponding for example to a pair on condition and the second condition corresponding to a pair off condition. A cam is mounted within the housing, the cam defining a cam surface against which a cam follower bears. Displacement of the cam causes displacement of the cam follower to actuate the switch.
In one known switch described in U.S. Pat. No. 5,665,947, the cam is defined by a side surface of an axially displaceable shaft. The cam follower is in the form of a ball which is biased against the side of the shaft and a switch operating member which is pushed outwards relative to the housing so as to protrude into contact with the switch. The arrangement is such that a mechanical snap action mechanism is achieved which locks the cam structure in position so as to maintain the switch operator in an extending position after axial displacement of the shaft either as the result of the pulling of the shaft outwards relative to the housing as a result of tension being applied to a cable connected to the shaft or as a result of that cable being severed. Thus the shaft is displaceable from an intermediate position in which the switch assumes one of the first and second conditions and positions displaced in either direction relative to that intermediate position in which the switch is in the other of the two conditions.
The arrangement described in U.S. Pat. No. 5,665,947 works satisfactorily but the overall size of the mechanism is relatively large given that components displaced as a result of axial movement of the shaft are located to the side of that shaft. Furthermore, although the described mechanism is claimed to provide a snap action, great care must be taken in setting up the various components so that the system always operates reliably. It is of course important that if the shaft is displaced to a position in which an associated machine is disabled by for example pulling gently on the rope release of the rope does not result in the shaft returning to a position in which the machine is re-energized.
International Patent Application No. WO97/20334 describes a switch mechanism in which axial displacements of the shaft connected to the rope are used to displace a pivotal lever relative to a cam surface defined by the switching mechanism housing. This effectively amplifies the magnitude of axial displacements of the shaft so as to make it easier to arrange an over centre mechanism to rapidly rotate the cam as soon as a relatively small displacement of the shaft has occurred. The positioning of a pivotally mounted lever and a cam surface to one side of the actuator shaft does however require a relatively large housing to accommodate all of the components.
In one aspect the present invention provides a switch mechanism which can be used in association with a rope switch arrangement and which is both compact and reliable.
According to the present invention there is provided a switch mechanism comprising a housing, a switch mounted within the housing, the switch being actuable to switch between first and second conditions, a cam mounted within the housing to rotate about a predetermined cam axis, the cam defining a cam surface, a cam follower mounted within the housing, the cam follower bearing against the cam surface such that rotation of the cam causes displacement of the cam follower to activate the switch, and an actuator shaft mounted to be axially displaceable within the housing, the actuator shaft being mechanically coupled to the cam such that axial displacement of the shaft causes the cam to rotate about its axis, wherein the actuator shaft is positionable in an intermediate axial position in which the cam is rotatable to a predetermined rotational position such that the cam follower causes the switch to assume the first condition, displacement of the actuator shaft from the intermediate position when the cam is in the predetermined position rotates the cam such that the cam follower causes the switch to assume the second position, the cam axis is transverse to the displacement direction of the actuator shaft, and the actuator shaft defines first and second abutment surfaces, the first abutment surface bearing against and causing rotation of the cam when the actuator shaft is displaced from the intermediate position in one direction, and the second abutment surface bearing against and causing rotation of the cam when the actuator is displaced from the intermediate position in the other direction.
An end of the actuator shaft adjacent the cam may define first and second arms which extend on opposite sides of the cam, the first arm defining the first abutment surface and the second arm defining the second abutment surface. Each arm may define further abutment surfaces, the further abutment surfaces obstructing rotation of the cam to the predetermined rotational position unless the actuator shaft is in the intermediate position. The arms may be defined by a fork member which is separable from but axially locked to a shaft member. The cam rotation axis is preferably aligned with an axis along which an actuator shaft is displaceable. Thus a very compact overall mechanism can be provided with all the active components in line.
A snap-action operation may be achieved by providing a member which is biased against the cam in a direction transverse to the cam axis, and arranging that the member bears against the side of the cam shaped such that when the cam is in the predetermined rotational position the biasing direction is aligned with the cam axis, and such that after rotation of the cam away from the predetermined rotational position the member applies a torque to the cam in a direction to increase that direction.
Given that the cam axis is transverse to the actuator shaft axis, one end of the cam may extend outside the housing to enable that end to be gripped to apply torque manually to rotate the cam.
The housing may comprise three sections, each section receiving a respective one of the actuator shaft, the cam and the switch. The actuator shaft extends into the section receiving the cam and the cam follower extends from the switch into the section receiving the cam.
This and other aspects of the invention are not intended to define the scope of the invention for which purpose claims are provided. In the following description, reference is made to the accompanying drawings which form a part hereof, and which there is shown by way of illustration, and not limitation, preferred embodiments of the invention. Such embodiments do not define the scope of the invention and reference must therefore be made to the claims for this purpose.